Conceptual Physics (12th Edition)

by
Hewitt, Paul G.

Published by
Addison-Wesley

ISBN 10:
0321909100

ISBN 13:
978-0-32190-910-7

Chapter 7 - Think and Discuss: 112

Answer

The string tension is 100 N because a single string supports mass A in equilibrium, so the upward tension is canceled by its downward weight, mg. In this ideal situation, tension is the same everywhere in the string.
Block B (along with the massless pulley) is supported by two strands of string, each with tension of 100 N and directed upward. Its weight and mass are twice that of Block A.